With conventional screen bowl type centrifugal separators, when a feed stock consisting of crystalline solid matters and a solvent is supplied to the inside of the bowl, the feed stock is separated into the crystals, which are the object to be treated, and the solvent by the centrifugal force, the crystals being settled onto the inner circumferential surface of the bowl, conveyed by the screw conveyor which is provided with a minute difference in speed between it and the bowl, and subjected to deliquidization at the tapered part provided on one side of the bowl.
The deliquidized crystals generally have impurities produced in the process of manufacture thereof, and the solvent itself deposited on their surfaces, and in order to clean these surplus deposits, a screen part has conventionally been provided on the inner circumferential side of the bowl following the tapered part, with cleaning nozzles which jet spout the cleaning liquid toward the pertinent portion being provided in the hub of the screw conveyor for carrying out cleaning by jet spouting the cleaning liquid all over the crystals which are being conveyed in the screen part. For example, refer to the patent literature 1 (Japanese Laid-Open Publication No. 2000-325833).
However, with the screen bowl type centrifugal separator as mentioned above, the crystals will not be conveyed by the conveyor in the clearance in the radial direction that is formed between the outer circumferential edge of the flight of the screw conveyor and the inner circumferential surface of the screen part, the residual layer of crystals being pressed against the outer circumferential edge of the flight to be firmly tightened without being moved in a long period of time of operation.
The crystals forming such a residual layer present a problem that they impede not only the penetration of the cleaning liquid, but also the movement of themselves for replacement with new residual layer crystals. Such a condition is generally called screen clogging. To eliminate the condition of clogged screen, it has been required to temporarily stop the supply of the feed stock, and instead, supply the cleaning liquid for a certain period of time. Thus, while the supply of the feed stock is stopped, the production is interrupted, which has been a factor of lowering the productivity.
Further, there has been a problem that, when the cleaning liquid is applied to all over the whole of the crystals which are conveyed by the flight, a part of the crystals is caused to leak in the screen part by the amount which is in proportion to that of the cleaning liquid passing through the layer of the crystals, and the amount which is in proportion to the mesh size of the screen.
Being developed in view of the problems of the prior art as mentioned above, the present invention is intended to provide a screen bowl type centrifugal separator which can not only eliminate the problem of reduction in productivity resulting from the occurrence of clogging due to the object to be treated, such as crystals, in particular, in the screen part, which is the critical portion of the screen bowl type centrifugal separator, but also minimize the amount of leakage of the object to be treated in the screen part.